Electromagnetic beams can serve as “tweezers,” enabling small objects to be accelerated, manipulated, or trapped using light alone. Optical tweezers were first introduced in 1970, using a laser beam to trap dielectric beads in lower-refractive-index media. Upon interaction with the laser, the bead was both accelerated in the direction of the beam and drawn toward the regions of high optical intensity.
Optical tweezers are a powerful means of probing and controlling micrometer-scale objects. In the biosciences, for example, optical tweezers have been used for bacterial trapping as well as noninvasive manipulation of organelles and filaments within individual living cells. They have also been used to study bio-molecular systems and the physics of molecular motors, ranging from kinesin and myosin to the polymerases involved in DNA transcription and replication. Optical traps have further enabled cooling of neutral atoms as well as translation, rotation, and assembly of relatively large nanowires and nanoparticles.
Despite these advances, optical trapping and manipulation of individual particles with sizes smaller than the wavelength of light remains a considerable challenge. The problem is inherent to the light beam itself. Optical trapping typically uses light in the visible spectrum (i.e., wavelengths between 400 and 700 nanometers) so that the specimen can be seen as it is manipulated. Due to the diffraction limit of light, the smallest space in which optical tweezing can trap a particle is approximately half the wavelength of the light beam; in the visible spectrum, this is about 200 nanometers (nm). If the specimen in question is much smaller than 200 nm, only very loose control of the specimen is possible since, relative to its size, the specimen is being trapped in a much larger potential well.
Furthermore, the optical force that light can exert on an object diminishes as the size of an object decreases. More particularly, in the Rayleigh regime (i.e., particle size smaller than the wavelength of light), optical forces on spherical particles scale with the third power of the particle's radius. As a consequence, optical forces diminish very quickly as particle size is reduced. Since the diffraction limit constrains the achievable intensity gradient, overcoming this reduction in force typically requires an increase in the illuminating optical intensity. But there are constraints to increasing intensity; in particular, increased intensity can damage the sample. It has been predicted, for example, that a 1.5 W laser beam could trap particles between 9 and 14 nm in diameter, depending on the refractive index of the particle. But such high optical powers would rapidly burn the particle.
Researchers have tried to circumvent this size limitation by tethering nano-sized molecular specimens to micrometer-scale dielectric beads that can be stably trapped and manipulated. The problem with such an approach is that a molecule might behave quite differently when tethered to what is effectively giant anchor than it would when un-tethered.
Recently, a technique called “plasmonic” tweezing has been used to extend conventional optical trapping to the sub-optical-wavelength regime. Plasmonic traps rely on excitation of surface plasmon-polaritons, which result from the coupling of light with the mobile conduction electrons at the interface of a conductor and insulator. That is, when light interacts with these mobile electrons, the light is scattered and sculpted into electromagnetic waves called “plasmon-polaritons.” These oscillations have a very short wavelength compared to visible light, enabling them to trap small specimens more tightly than is otherwise possible.
These electromagnetic modes are capable of confining light beyond the diffraction limit and are characterized by an exponential decay of electromagnetic fields away from the interface. These properties are very important for trapping applications; the former property significantly reduces the trapping volume, while the latter enhances the resulting optical forces due to the strong field gradient.
Several recent studies have demonstrated the feasibility of plasmonic optical trapping. In 2009, it was shown that plasmonic nano-antennas can trap 200 nm polystyrene particles using 300 mW (0.01 mW/μm2) of illumination power. In 2011, trapping and rotation of 110 nm polystyrene beads were achieved using plasmonic nano-pillars with an illumination intensity of 10 mW/μm2. More recently, trapping of 20 nm polystyrene particles was achieved within a plasmonic nano-cavity formed by a nano-pore and double nano-hole aperture. These demonstrations combined the plasmonic trap with “self-induced back action trapping,” allowing the required illumination power to remain below 10 mW.
In the biosciences, nano-photonic and plasmonic structures have enabled optical trapping of λ-DNA molecules and a single bovine serum albumin molecule with a hydrodynamic radius of 3.4 nm. Theoretical studies have shown that optical trapping of particles as small as 10 nm is possible within silicon slot waveguides and hybrid plasmonic waveguides. However, efficient trapping of sub-10-nm particles still remains a considerable challenge.